The present invention relates generally to integrated circuits and, more particularly to amplifiers having a low voltage CMOS differential amplifier input stage.
Today, electronic systems encompassing operational amplifiers generally have lower operating voltage supplies than they have in the past, mainly do to low power battery sources. Moreover, power supply voltages requirements continue to decrease, while dynamic range requirements remain essentially constant. Fortunately, various manufacturing processes for integrated circuits make it possible to create rail-to-rail differential input stages.
Although the various types of input stages operate from a single supply voltage source, the low voltage limit for amplifier operation differs for each type of input stage and each integrated circuit manufacturing process. Present op amp input stage designs exhibit voltage operation limits that hinder their application in products powered by batteries having an end life of near one volt. For example, an op amp using complimentary bipolar transistor differential pairs amplifying signals near positive and negative supplies has low operating voltage limitations imposed by standard transistor base to emitter voltage drops.
One conventional solution for rail-to-rail input stages has been the use of depletion-mode MOSFETs to provide amplification of the differential input. FIG. 1 is a schematic diagram showing a prior art input stage 100 for a low voltage operational amplifier. The input stage 100 includes a differential input signal VIN coupled to the gates of two N-channel depletion-mode MOSFETs 2 and 4. The drain of MOSFET 2 is coupled to one terminal of current source 6, and the drain of MOSFET 4 is coupled to one terminal of current source 8. The second terminals for both current sources 6 and 8 are coupled to operating potential VCC. Both source terminals of MOSFETs 2 and 4 are coupled to one terminal of current sink 10, while the other terminal of current sink 10 is coupled to ground reference. The bulk, or well, terminals of both MOSFET 2 and MOSFET 4 are also coupled to the ground reference.
The differential pair of MOSFETs 2 and 4 receives the input signal VIN and provides a differential output current from the drain terminals of MOSFETs 2 and 4 at inputs 14 and 16 to the rest of the system. While the prior art input stage 100 provides an input trans conductance, input stage 100 has limited uses. For example, input stage 100 requires the input NMOS transistors 2 and 4 to have a particular combination of threshold voltage and bulk concentration to have proper common mode range and function correctly. This causes the process requirements for input stage 100 to be strictly defined so that the common mode input range is maximized.
In view of the forgoing, what is needed is a versatile operational amplifier input stage that can be used in a variety of applications powered from battery sources. In addition, the amplifier input stage should allow near rail-to-rail performance and increased design flexibility over that provided by prior art input stages.
The present invention addresses these needs by providing a low voltage rail-to-rail CMOS input stage. In one embodiment, a low voltage operational amplifier input stage is disclosed. The input stage includes a differential pair of P-channel metal oxide semiconductor field effect (PMOS) transistors, which produces a differential current. The input stage further includes two N-channel depletion-mode metal oxide semiconductor field effect (NMOS) transistors, coupled to the bulk terminals of the differential pair of PMOS transistors, for receiving an input signal. The depletion-mode NMOS transistors further act as source follower devices to drive the bulk terminals of the differential pair of PMOS transistors.
In another embodiment, a method for providing an output signal from an input stage of a low voltage operational amplifier is disclosed. The method includes providing an input signal to two NMOS transistors coupled to bulk terminals of a differential pair of PMOS transistors. The method further includes providing first and second alternating current signals using the differential pair of PMOS transistors.
An application specific integrated circuit (ASIC) having an input stage for a low voltage operational amplifier input stage is disclosed. The ASIC includes a differential pair of P-channel metal oxide semiconductor field effect (PMOS) transistors, which produces a differential output current. The ASIC further includes two N-channel depletion-mode metal oxide semiconductor field effect (NMOS) transistors, coupled to the bulk terminals of the pair of PMOS transistors, for receiving an input signal. The depletion-mode NMOS transistors further act as source follower devices to drive the bulk terminals of the differential pair of PMOS transistors.
In yet another embodiment, a low voltage operational amplifier input stage is disclosed. The input stage includes a differential pair of NMOS transistors, which produces a differential current. The input stage further includes two depletion-mode PMOS transistors coupled to the bulk terminals of the differential pair of NMOS transistors, for receiving an input signal. The depletion-mode PMOS transistors further act as source follower devices to drive the bulk terminals of the differential pair of NMOS transistors.
In a still further embodiment, a low voltage operational amplifier input stage is disclosed. The input stage includes a differential pair of NMOS transistors, which produces a differential current. The input stage further includes two JFET transistors coupled to the bulk terminals of the differential pair of NMOS transistors, for receiving an input signal. The JFET transistors further act as source follower devices to drive the bulk terminals of the differential pair of NMOS transistors.
Advantageously, the present invention provides a versatile operational amplifier input stage that can be used in a variety of applications, including applications powered by low power battery sources. Moreover, the input stage of the present invention provides essentially rail-to-rail performance and increased design flexibility over that provided by conventional input stages.